1. Field of Invention
The present invention relates to a projector which divides light from a light source into a plurality of partial light beams, which converts the plurality of partial light beams into one type of polarized light beam polarized in substantially the same direction by a polarization conversion element, which changes the polarized state of the polarized light beam by an electro-optical device, which selects a state by a polarization selection element to form an optical image according to image information, and which enlarges and projects the optical image.
2. Description of Related Art
Recently, attention has been focused on projectors using a reflective-type liquid crystal device. In such a reflective-type liquid crystal device, the pixel density can be increased by forming a structure, such as a transistor, for driving liquid crystal under a reflecting mirror. Therefore, the reflective-type liquid crystal device has the advantage of realizing a clear projected image with high resolution, compared with the case where a transmissive liquid crystal device is used.
In addition, in projectors using an electro-optical device, such as a liquid crystal device, in order to reduce the size of the entire device while realizing a bright projected image without display nonuniformity, the use of an integrator optical system or a polarization conversion element has been proposed (Japanese Unexamined Patent Application Publication No. 8-34127, and Japanese Unexamined Patent Application Publication No. 10-232430, etc.). In the integrator optical system, light from a light source is divided by a light beam dividing optical element into a plurality of partial light beams to form a plurality of light source images, the light source images are considered as dummy light sources, and light from the plurality of light source images is superposed on a liquid crystal panel, whereby illumination light having a uniform intensity distribution can be obtained. In the polarization conversion element, light from a light source is divided into a plurality of partial light beams to perform polarization conversion and then, the light is superimposed on a liquid crystal device, whereby illumination light polarized in the same direction is obtained.
For this reason, it is thought that a brighter projected image with high resolution and without display nonuniformity can be realized if the integrator optical system and the polarization conversion element are used in combination in the projector using the reflective-type liquid crystal device.
When a reflective-type liquid crystal device utilizing a polarization mode as a display mode is used in a projector, a polarization selection element (for example, a polarization beam splitter) for spatially separating and selecting light of different polarization states is generally used, but the polarization selecting characteristic of the polarization selection element has strong incident-angle-dependency. More specifically, in the case where a plane of incidence including a nearly central axis of the incident light and a normal line of a polarization selection surface of the polarization selection element is defined, if the incident angle of light is increased in a plane perpendicularly intersecting the plane of incidence, the polarization selectivity is substantially reduced. Since this phenomenon greatly depends on a geometrical positional relationship between a polarization selection surface and the light entering there, it is very difficult to prevent the substantial reduction in the polarization selectivity. On the other hand, if the incident angle of light is increased at the plane of incidence, the polarization selectivity is also reduced, but the degree of reduction is relatively small as compared to that in the plane perpendicularly intersecting the plane of incidence, and the reduction in the polarization selectivity can be prevented by arranging the configuration of the polarization selection surface. Therefore, in order to at least improve the polarization selectivity of the polarization selection element, it is important to reduce the incident angle of light in the plane perpendicularly intersecting the plane of incidence as much as possible, for example.
In addition, the optical system employing the integrator optical system or the polarization conversion element, by reason of its optical process, cannot avoid the phenomenon in which the angular distribution of the incident angle of illumination light expands.
For this reason, in the case where the integrator optical system and the polarization conversion element are used in combination in the projector using the reflective-type liquid crystal device, since the incident angle of light entering the polarization selection surface is increased, the polarization selectivity of the polarization selection surface is reduced, causing a problem in that light utilization efficiency is reduced and non-uniform brightness occurs.
It is one object of the present invention to at least provide a projector which can realize a bright projected image with high light utilization efficiency and high quality while combining a reflective-type liquid crystal device and an integrator optical system or a polarization conversion element.
The projector according to the present invention achieves at least the above object by, for example, arranging a direction of polarization beam separation and characteristics of a light beam dividing optical element.
(1) The projector according to one exemplary embodiment of the present invention is a projector including a light beam dividing optical element for dividing light from a light source into a plurality of partial light beams; a polarization conversion element for converting the plurality of partial light beams into one type of polarized light beam polarized substantially in the same directions; an electro-optical device for modulating an illumination light beam emitted from the polarization conversion element; a projection lens for projecting light modulated by the electro-optical device; and a polarization selection surface for selecting light of a predetermined polarized component included in the illumination light beam and emitting the light toward the electro-optical device, and for selecting light of a predetermined polarized component in the light modulated by the electro-optical device and emitting the light toward the projection lens. In the projector, when a plane of incidence including a normal line of the polarization selection surface and the central axis of the illumination light beam is assumed, the direction parallel to the plane of incidence and perpendicularly intersecting the central axis is defined as the X-axis direction, and the direction perpendicularly intersecting the plane of incidence is defined as the Y-axis direction, the direction of polarization beam separation by the polarization conversion element is the X-axis direction.
According to the exemplary embodiment as described above, the polarization beam separability of the polarization selection surface has strong incident-angle-dependency to an incident light beam. In particular, when an incident angle of light is increased in the Y-axis direction perpendicularly intersecting the plane of incidence, the polarization selectivity is remarkably reduced. On the other hand, in the polarization conversion element, since two types of polarized light beams polarized in different directions are produced from the partial light beams, the width of each partial light beam substantially doubles in the direction of separation, and the angular distribution of the light expands. Thus, in order to improve the polarization selectivity in the polarization selection element, it is important to consider the incident-angle-dependency of the polarization selectivity and the spread of the angular distribution of the light incident thereon.
According to this exemplary embodiment, since the direction of polarization beam separation in the polarization conversion element is the X-axis direction, an increase in the incident angle of light in the Y-axis direction incident on the polarization selection surface can be restrained. Thus, the polarization selectivity can be maintained in a relatively high state, making it possible to realize a bright projected image having a high contrast ratio.
(2) As the electro-optical device, for example, a reflective-type liquid crystal device disposed at a position on which either light transmitted or reflected by the polarization selection surface is incident, modulating the incident light, and emitting the modulated light from the plane of incidence of the light, may be adopted.
(3) The light beam dividing optical element may preferably be configured so as to narrow the spacings of the plurality of light source images in the Y-axis direction.
That is, since the increase in the incident angle of light in the Y-axis direction can be further restrained by narrowing the spacings of the light source images in the Y-axis direction, the polarization selectivity of the polarization selection surface can be maintained in a very high state, making it possible to realize a very bright projected image having a high contrast ratio.
(3-1) As the light beam dividing optical element, a rod for reflecting light incident from an incident end surface at plural pairs of reflection surfaces, dividing the light according to differences in reflection positions, and emitting the light as a plurality of partial light beams from an emission end surface, can be adopted.
As the rod, a solid one (solid rod) consisting of light-guiding material, or a hollow one (hollow rod) having a light reflecting surface formed on the inside surface of a cylindrical member can be adopted. In the case of the solid rod, light is totally reflected by the reflecting surface without optical loss, so that the light utilization efficiency can be further increased. In the case of the hollow rod, since light incident from the incident end surface reaches the emission end surface via an air layer in the rod, uniform illumination light can be realized even if the size between the incident end surface and the emission end surface is set to be relatively short, and further, the hollow rod is manufactured more easily than the solid rod.
When the solid rod or the hollow rod is adopted, it may include at least two sets of reflecting surfaces opposing in the X-axis direction and in the Y-axis direction, and the cross section of the rod can be formed into a polygon of a tetragon or more, such as an octagon, a dodecagon, or the like.
However, if the light transmission efficiency from the light source to the light beam dividing optical element is considered, since the light incident on the light beam dividing optical element from the light source has a substantially circular cross section, the incident end surface of the rod may preferably be formed in a square shape. In addition, if the illuminating efficiency to the subsequently disposed electro-optical device is considered, since an image formed on the emission end surface of the rod is superimposed on a display area of the electro-optical device that is one area to be illuminated, the emission end surface of the rod may preferably have the shape substantially similar to the shape of the display area of the electro-optical device.
In the case of adopting the above-described rod as the light beam dividing optical element, the spacings of the light source images in the Y-axis direction can be narrowed by disposing the rod so that a spacing of a pair of the reflecting surfaces opposing in the Y-axis direction is gradually widened from the incident end surface toward the emission end surface.
Furthermore, the rod may be disposed so that a spacing of a pair of reflecting surfaces opposing in the X-axis direction is gradually narrowed from the incident end surface toward the emission end surface of the rod. In this case, since the disposition spacings of the light source images in the X-axis direction can be widened, the spacings between the polarization beam separation films and the reflecting films of the polarization conversion element can be set in sufficient consideration of the sizes of the light source images. Thus, the polarization conversion efficiency in the polarization conversion element can be increased, and consequently, making it possible to increase the light utilization efficiency in the projector.
(3-2) As the light beam dividing optical element, a lens array composed of a plurality of condenser lenses aligned in the X-axis direction and the Y-axis direction can be also adopted.
In this case, it is possible to narrow the spacings of the plurality of light source images in the Y-axis direction by designing the light collecting characteristics of the plurality of condenser lenses. As the condenser lenses constituting the lens array, hologram lenses or diffraction lenses for condensing light by a holographic effect or diffraction can be also adopted in addition to a general lens.
In addition, since the images formed on the condenser lenses of the lens array are superimposed on a display area of the electro-optical device that is one area to be illuminated, the condenser lenses may preferably have the shapes substantially similar to the shape of the display area of the electro-optical device. This can increase the illumination efficiency.
In addition, a part of or all of the plurality of the condenser lenses constituting the lens array may preferably be a decentered lens.
That is, since the light source images can be formed at positions other than the physical centers of the condenser lenses by forming a part of or all of the condenser lenses with the decentered lens, the spacings of the plurality of light source images formed on a virtual plane can be freely controlled.
(4) When the lens array is adopted as the light beam dividing optical element, a reducing optical system may preferably be disposed on an optical path provided between the light source and the polarization conversion element. By reducing the overall cross sectional size of the illumination light with the reducing optical system, the increase in the incident angle of light in the Y-axis direction can be further restrained.
By the disposition of such a reducing optical system, the overall cross sectional size of the illumination light can be reduced in the Y-axis direction. For this reason, the increase in the incident angle of light in the Y-axis direction can be further restrained, and the polarization selectivity of the polarization selection surface can be maintained in a very high state. Therefore, it is possible to realize a very bright projected image having high contrast ratio. In addition, since the overall diameter of the light beam illuminating the area to be illuminated can be reduced, an expensive lens having the small F-number does not have to be adopted as the projection lens. Therefore, a reduction in the cost of the projector can be realized.
In this case, not only the cross sectional size in the Y-axis direction, but also the cross sectional size in the X-axis direction may be reduced. In this case, it is possible to maintain the polarization selectivity of the polarization selection surface in a higher state.
Such a reducing optical system can be constituted by at least one convex lens disposed on one of the incident side and the emission side of the lens array, and at least one concave lens disposed on the incident side of the polarization selection element. In this case, in the case where only the cross sectional size in the Y-axis direction of the illumination light beam is reduced, cylindrical lenses can be used as the concave lens and the convex lens. While the convex lens and the concave lens can be constituted by one lens member, respectively, they may preferably be a combined lens formed by a combination of a plurality of lenses if the reduction in the optical aberration is considered.
(5) In the above projector, a reducing optical system for reducing the cross sectional size of the illumination light in the Y-direction can be disposed between the polarization conversion element and the polarization selection element.
While the reducing optical system can be constituted by one concave lens, it can be also constituted by a combined lens formed by a combination of a plurality of lenses. If the reduction in the optical aberration is considered, the combined lens may preferably be adopted. In this case, cylindrical lenses can be used as the convex lens and the concave lens.
By the adoption of such a reducing optical system, the same advantages as in the case of (4) can be also obtained.
In addition, in this case, not only the cross sectional size of the illumination light in the Y-axis direction, but also the cross sectional size in the X-axis direction may be reduced. In this case, general axisymmetric curved lenses can be used as the concave lens and the convex lens.
The convex lens and the concave lens constituting a series of the above reducing optical systems may be hologram lenses or diffraction lenses for condensing light by a holographic effect or diffraction, in addition to general lenses having surfaces formed into curved shapes.
(6) As the polarization conversion element, a polarization conversion element including a polarization beam separation film for transmitting one polarized light beam and for reflecting the other polarized light beam in two types of polarized light beams, a reflecting film for reflecting the other polarized light beam, and a retardation film for unifying the directions of polarization of the two types of polarized light beams in order to unify the directions of emission of the two types of the polarized light beams, may preferably be adopted.